Field of the Invention
The present invention relates to discrete resistors. More particularly, the invention relates to resistive components with high micro-linearity characteristic.
Description of the Related Art
A conventional resistor which has been used for variable resistors of various sensors has at least two layers consisting of a lower layer and an upper layer. The exposed surface of the upper layer functions as a face on which a slider is slid. The upper layer and the lower layer contain conductive particles such as carbon black, embedded in a binder resin, where the upper layer usually has a larger resistivity than the lower layer. The upper layer is designed to withstand continual friction by the slider without effecting the electric characteristic of the resistor throughout its product life. A resistor used for a high-precision sensor is required to have an excellent micro-linearity characteristic. The graph shown in FIG. 4 illustrates how the micro-linearity characteristic of a resistor varies with the central particle size of the carbon fiber (CF), and the amount of carbon black (CB) and carbon fiber (CF) contained in the upper layer.
Generally, the factor with the largest change in signal-to-noise ratio has the greatest influence on micro-linearity. As indicated in the graph, the amount of carbon black contained in the upper layer has the largest influence on micro-linearity.
FIG. 5 is a graph showing the relationship between the resistivity of the upper layer and the micro-linearity characteristic when the central particle size range of the carbon fiber contained in the upper layer is 1.4 xcexcm or 8.7 xcexcm. As described later, a smaller variation results in better micro-linearity characteristic. As shown in FIG. 3, within a certain range, a lower resistance relative to the lower layer results in improved micro-linearity characteristic.
In known resistor with a similar configuration (i.e., two or more layers where the upper layer is set to a higher value than that of the lower layer) there is a problem in achieving adequate micro-linearity for such applications as high-precision variable resistors. Additionally, the product life of many know variable resistors is shortened because they contain no carbon fibers in the upper layer.
The object of the invention is to provide a resistor excellent in micro-linearity characteristic and further, durability to sliding as well as a high-precision variable resistor using the resistor and having a long life.
To achieve the foregoing and in accordance with the objects of the invention, a resistor is made by laminating at least two resistive layers, where these resistors have conductive particles held in a binder resin. The resistors are laminated such that a top resistor covers a bottom resistor, and a surface of the top resistor is exposed. The resistivity of the top resistor is made smaller than that of the bottom resistor. The top resistor contains carbon fiber and carbon black, where the central particle size of the carbon fiber ranges from 3.5 to 9.0 xcexcm.
In some embodiments the top resistor includes carbon fiber and carbon black. The central particles size of the carbon fiber contained in the top resistor can be equal to or smaller than that of the carbon fiber contained in the bottom resistor. In other embodiments the bottom resistor may contain carbon fibers in the range of 16 to 20% by volume, and/or have a resistivity less than, but at least equal to one tenth of the resistivity of the top resistor. The bottom resistor may also have a maximum surface roughness of 0.5 xcexcm.
In other embodiments, the resistor made according to the present invention may be used as a variable resistor where additional aspects include making the central particle size of the carbon fiber contained in the top resistor equal to or smaller than that of the carbon fiber contained in the second resistor.